Field of the Invention
The invention concerns an inching and braking system for a hydraulically powered apparatus.
In the case of heavy utility vehicles, such as e.g. wheel loaders, stacker trucks etc., it is known to utilise the drive system for decelerating the apparatus in addition to the brake system per se in order to produce a braking effect. Braking by means of the drive system is generally referred to as "inching". Splitting the braking force between brake system and drive system has the advantage that wear to the brakes of the apparatus may be reduced quite considerably inasmuch as the drive system may primarily be utilised for deceleration at minor changes of speed.
FIG. 1 is a schematic representation of the drive system of a utility vehicle, the driven axle 100 of which is driven by means of a hydrostatic drive 102. The latter comprises for example a hydromotor 104 having an adjustable displacement volume and supplied with hydraulic fluid by a variable displacement pump 106 which, in turn, is driven by an engine 108. In order to generate a braking effect with the aid of the drive system it is possible to intervene either directly at the driven axle 100, i.e. in the area of the unit of hydromotor/variable displacement pump 104, 106 to be precise, or in the area of the combustion engine 108, in which latter case it is possible e.g. to control the camshaft control or an engine brake.
A conventional inching and braking system which may be used in the like systems is represented in FIG. 2. Control of the inching and braking system is carried out through actuating means, e.g., a brake pedal 112 acting. on an inching and braking valve 114 through an actuation rod. At the inching and braking valve 114 a receiver pressure pS provided by an accumulator or a suitable pump is applied. Upon actuation of the brake pedal 112 and the resulting displacement by the angle .alpha., the inching and braking valve 114 delivers a braking pressure PB which is conveyed to a brake unit 116 having, for example, the form of a multiple disk brake. The brake unit 116 is biased through a readjusting spring or a biasing valve (not shown) such that the brake will operate only after a minimum pressure of e.g. 9 bar is exceeded. Upon release of the brake pedal 112 the connection between the accumulator and the brake unit 116 is interrupted and the braking pressure p.sub.B is relieved into a tank T, whereby the brake unit 116 is depressurised.
From the brake line conveying the braking pressure p.sub.B a control line is branched off and routed to a converter or inching valve 118 whereby the drive system 102 of the apparatus can be controlled. In dependence on the applied control pressure corresponding to the braking pressure p.sub.B, a flow Q, for example supplied by a gear pump, is converted with a pressure p into a control pressure or inching pressure p.sub.I, which may be utilised e.g. for controlling a driving direction reversing valve and thus the operating cylinder of the variable displacement pump 106 of the drive system 102.
As is indicated by the dot-dash lines in FIG. 3, the known inching and braking valve 114 is designed such that the braking pressure PB increases with an increasing displacement of the brake pedal 112 (pedal travel .alpha.), so that the brake unit 116 causes greater deceleration of the vehicle as the pedal travel .alpha. increases. As shall be explained in detail herebelow, the characteristic of the inching and braking valve 114 is commonly selected such that in a first portion a shallow increase of the braking pressure p.sub.B occurs concurrently with the pedal travel .alpha., whereas in a subsequent second portion the braking pressure p.sub.B increases more sharply. The flat portion of the PB characteristic curve is associated with the inching operation, whereas the sharply rising portion of the pB characteristic curve is associated with the actual braking operation by the brake unit 116 (biased axles).
It can moreover be taken from FIG. 3 that the inching pressure p.sub.I (continuous line in FIG. 3) drops from a maximum value to a value approaching 0 as pedal travel .alpha. increases. In other words, while the brake pedal, 112 is not actuated, the drive unit 102 is supplied by the inching valve 118 with the maximum inching pressure p.sub.I which in turn is dependent on the rotational speed, with the inching valve 118 acting similar to a pressure control valve and opening a connection towards the tank T if the pressure p.sub.I is greater than the preselected maximum inching pressure. Concurrently with an increasing pedal travel .alpha., the braking pressure PB and correspondingly the control pressure applied to the inching valve 118 rise, whereby in accordance with the characteristic curve of the inching valve 118 the inching pressure p.sub.I drops and the output of the variable displacement pump of the drive unit 102 is reduced and the apparatus is decelerated as a result of this control of the drive unit 102.
The inching pressure p.sub.I attains its minimum value (0) approximately at the time when the braking pressure reaches the range in which the braking operation by the brake unit 116 starts. Due to its bias, the brake unit 116 operates only when the limit value predetermined by the bias (e.g., 9 bar) is exceeded.
Concerning the construction of the inching and braking valve 114 and of the inching valve 118, reference is made to the older application P 195 13 805 by the same applicant, wherein a conventional inching and braking unit is already described. The subject matter of this older patent application is also included in the disclosure of the present patent application by way of reference.
The characteristic curves of the inching and braking valve 114 and of the inching valve 118 represented in FIG. 3 may only be obtained if these two components are precisely harmonised with each other. Even at minute deviations from the preselected configuration particularly in the transitional range in which the inching pressure has dropped to nearly 0, while the braking pressure applied to the brake unit 116 reaches its threshold (spring bias) in which the brake unit 116 intervenes, control variations may occur owing to the interference of the inching deceleration effect with the braking effect of the brake unit 116.
As a possible result, the operator will not be enabled to operate the apparatus with the required sensitivity over the entire inching range.